Flow reactor approach for the facile and continuous synthesis of efficient Pd@Pt core-shell nanoparticles for acceptorless dehydrogenative synthesis of pyrimidines from alcohols and amidines

Article abstract of DOI:10.1016/j.apcata.2021.118158

Carbon supported Pd@Pt core-shell nanoparticles catalyst was prepared in a flow reactor toachieve enhanced catalytic activities with low Pt loading for the acceptorless dehydrogenative synthesis of pyrimidines. Spectroscopic (XAS analysis) and microscopic (HAADF-STEM) techniques reveled that the core-shell structure was formed by the applied preparation method. The Pd@Pt/PVP (polyvinylpyrrolidone)/C catalyst showed the activity for the three component one pot synthesis of pyrimidines through a series of consecutive reactions including oxidation of alcohols, C[sbnd]C, and C[sbnd]N coupling, followed by heterocyclization and dehydrogenation employing various primary alcohols, secondary alcohols, and amidines. The reaction mechanism on Pd@Pt/PVP/C catalyst was explored by comparison with the control experiments.

Full text of DOI:10.1016/j.apcata.2021.118158

Applied Catalysis A, General 619 (2021) 118158
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Flow reactor approach for the facile and continuous synthesis of efficient
Pd@Pt core-shell nanoparticles for acceptorless dehydrogenative synthesis
of pyrimidines from alcohols and amidines
,
Sharmin Sultana Polya *, Yuta Hashiguchib, Asima Sultanaa, Isao Nakamuraa,
,
Ken-ichi Shimizuc, Shunsaku Yasumurac, Tadahiro Fujitania *
a Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology Tsukuba, Ibaraki 305-8565, Japan
b Research Association of High-Throughput Design and Development for Advanced Functional Materials, Tsukuba, Ibaraki, 305-8565, Japan
c Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Carbon supported Pd@Pt core-shell nanoparticles catalyst was prepared in a flow reactor toachieve enhanced
catalytic activities with low Pt loading for the acceptorless dehydrogenative synthesis of pyrimidines. Spectro-
scopic (XAS analysis) and microscopic (HAADF-STEM) techniques reveled that the core-shell structure was
formed by the applied preparation method. The Pd@Pt/PVP (polyvinylpyrrolidone)/C catalyst showed the ac-
tivity for the three component one pot synthesis of pyrimidines through a series of consecutive reactions
Acceptorless dehydrogenative coupling
Multicomponent reaction
Pyrimidines
Flow reactor
Core-shell catalyst
–
–
including oxidation of alcohols, C C, and C N coupling, followed by heterocyclization and dehydrogenation
employing various primary alcohols, secondary alcohols, and amidines. The reaction mechanism on Pd@Pt/
PVP/C catalyst was explored by comparison with the control experiments.
1. Introduction
heterocycles synthesis with variations in oxidants, solvents, and starting
materials [42–46]. Transition metal based homogeneous catalysts have
Pyrimidines are important heterocycles in the synthesis of pharma-
ceuticals ingredients, agrochemicals, and various functional materials
[1–3]. They have structural moieties as in natural products and bio-
logically active molecules [4]. Pyrimidine is a key factor of some
important drugs used for the treatment of hyperthyroidism, acute leu-
kemia in children, and adult granulocytic leukemia [4–7]. In addition,
several other pyrimidines show wide-spread pharmacological activities
as antitumor, antibacterial, antifungal, antimalarial, analgesic, anti-
cancer, and anti-cholesterol drug [5,8–18]. Therefore, the development
of a green and sustainable method for the synthesis of pyrimidine de-
rivatives from easily available starting materials is desired.
been utilized for synthetic processes related to fine chemicals, such as
flavors and pharmaceuticals. Following these studies, several groups
have recently reported new synthetic methods using the acceptorless
dehydrogenative coupling (ADC) to obtain pyrimidines from easily
available starting materials. The methods employ methodology. Kempe
et al. [16,47] Kirchner et al. [48] Herbert et al. [49], Kundu et al. [50],
Adhikari et al. [51] have reported the synthesis of pyrimidines through
ADC condensation with a basic additive (KOtBu) using homogeneous
transition metal (such as Ir, Mn, and Ni) in one-pot multicomponent
reaction. Very recently, Pt/C catalyst in a recyclable heterogeneous re-
action system has been reported for the synthesis of pyrimidines from
alcohols and amidines under ADC method [26]. Compared with the
previous homogeneous catalytic system [14,34,47–51], the developed
system achieved a high activity, higher turnover number (TON), a wide
range of substrates scope and good reusability of the catalyst toward
pyrimidine synthesis (Scheme 1).
Over the past several decades, alcohols have been regarded as
effective and inexpensive substrates transformed into value-added het-
eroatom-containing chemicals [19,20]. Among the transformations, H2
evolving, acceptorless dehydrogenation (AD) reactions have become
–
–
promising for eco-friendly C C and C N bond formation reactions
using alcohols [21–41].
To enhance the catalytic activity and reduce the usage of Pt, alloying
Pt with Pd has been actively studied in several reports [52–67] and most
There are several reports of multicomponent reactions of
* Corresponding authors.
E-mail addresses: poly.sharmin@aist.go.jp (S.S. Poly), t-fujitani@aist.go.jp (T. Fujitani).
https://doi.org/10.1016/j.apcata.2021.118158
Received 24 February 2021; Received in revised form 4 April 2021; Accepted 12 April 2021
Available online 15 April 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

S.S. Poly et al.
Applied Catalysis A, General 619 (2021) 118158
Scheme 1. Multi-component synthesis of pyrimidines.
Fig. 1. DF-STEM-EDS mapping images (A-D), and DF-STEM image (E) with Pd shown in green and Pt in red, of Pd@Pt/PVP/C NPs synthesized by the flow process.
of the reported catalysts were found effective towards polymer elec-
Table 1
trolyte membrane fuel cells (PEMFC) [53,65,68–70]. In the present
CO adsorption data of the catalysts pre-reduced at 300 ◦C.
work, we design
a
bimetallic core-shell Pd@Pt/PVP (poly-
Catalysts
Pt
Pd
Pt dispersion
(%)
Surface area
vinylpyrrolidone)/C catalyst was prepared in a flow reactor, resulting in
a high efficiency and a synergistic effect between Pt and secondary metal
Pd. The purpose of the PVP as a capping agent is to control the particle
size and to prevent the agglomeration of nanoparticles [71–73].
Core-shell nanoparticles are also used as a template for the preparation
of hollow particles after removing the core either by dissolution or
calcination [74–76]. However, there are very few reports in organic
synthesis with core-shell catalysts [77,78]. In this study, our challenge is
to expand the uses of the core-shell catalyst for the synthesis of indus-
trially important chemicals from multicomponent in a one pot manner.
The high productivity of the desired heterocycle pyrimidines is a reflect
of durability and high activity of synthesized bimetallic core-shell
Pd@Pt/PVP/C catalyst with low loading of Pt and precisely control
the nanoparticles particle size in flow reactor.
content
(wt%)
content
(wt%)
(m2 gꢀ 1
)
Pt/C
5
0
11.93
12.65
29.46
31.24
Pd@Pt/PVP/
C
2.5
2.5
loading of Pd and Pt for 5 wt%. The details of the flow reactor and the
preparation method of Pd@Pt/PVP/C catalyst are in experimental sec-
tion (Fig. S1). Then the catalyst was characterized by various charac-
terization techniques. To directly observe the core-shell structure of the
supported nanoparticles, the dark field scanning transmission electron
microscopy (DF-STEM) images were obtained, and elemental mapping
with energy dispersive spectroscopy (EDS) was carried out. Fig. 1 shows
STEM images for Pd@Pt/PVP/C samples. In this case, the edge of the
sample was rich in Pt while the core was rich in Pd. From this result, we
can conclude that the structure of core-shell catalyst Pd@Pt/PVP/C had
been successfully synthesized.
2. Results and discussion
2.1. Characterization of Pd@Pt/PVP/C
The particle size of this Pd@Pt/PVP/C catalyst is 3.55 ± 0.62 nm
(Fig. S2). Furthermore, from the results of electron energy loss spec-
troscopy (EELS) line analysis (Fig. S3), the thickness of the Pt shell was
estimated to be approximately 0.5 nm, suggesting that the Pt shell
Pd@Pt core-shell NPs were synthesized using the flow-reactor pro-
cess with K2PdCl4 as a Pd precursor, H2PtCl6 as a Pt precursor, NaBH4 as
a reductant, and polyvinylpyrrolidone as a capping agent with a total
2

S.S. Poly et al.
Applied Catalysis A, General 619 (2021) 118158
Fig. 2. Pt L3-edge (A) XANES spectra and (B) EXAFS spectra of Pd@Pt/PVP/C before and after the pre-reduction at 300 ◦C.
Table 2
Curve-fitting analysis of Pt L3-Edge EXAFS of Pd@Pt/PVP/C.
Conditions
Shell
CN a
R /Å b
σ
/Å c
Rf (%)d
k range/Åꢀ 1
(R range/Å)
Number of
free parameters
Cl
Pd
Pt
Cl
Pd
Pt
1.2 ± 0.95
1.9 ± 0.70
4.6 ± 1.06
0.97 ± 0.85
2.3 ± 0.58
4.7 ± 0.70
2.09 ± 0.04
2.72 ± 0.01
2.71 ± 0.01
2.09 ± 0.05
2.73 ± 0.01
2.72 ± 0.01
0.095 ± 0.07
0.081 ± 0.05
0.081 ± 0.04
0.098 ± 0.08
0.070 ± 0.04
0.082 ± 0.03
As-synthesized
Pd@Pt/PVP/C
9.0
7.0
3–16
12
(1.6–3.4)
Pd@Pt/PVP/C
(after H2 reduction)
a
Coordination numbers.
b
Bond distance.
c
d
Debye-Waller factor.
Residual factor.
Table 3
Catalyst screening for the Synthesis of 2,4,6 triphenyl pyrimidine from 1-phenyl ethanol, benzyl alcohol and benzamidine.
Entry
Pd (wt%)
Pt (wt%)
Cat. amount (mol%)
GC yield (%)
1
–
5
1
95
42
68
17
81
93
84
2
5
–
1
3
–
2.5
–
2.5
2.5
2.5
1
4
2.5
2.5
2.5
2.5
1
5a
6b
7c
0.5
0.5
0.5
a
Pd@Pt/PVP/C prepared in batch process.
b
c
Pd@Pt/PVP/C prepared in flow process (with 1.1 mmol KOtBu).
Pd@Pt/C prepared in flow process.
consisted of two atomic layers. Even after pre-reduction under a H2 at-
mosphere at 300 ◦C, the core-shell structure is still stable and particle
size is almost similar to fresh catalyst (Fig. S4) [79]. Table 1 shows Pt
dispersion for the catalysts (pre-reduced at 300 ◦C) estimated by a
standard CO chemisorption method. The result shows that 5 wt% Pt/C
and 2.5 wt% Pd@Pt/PVP/C have similar Pt dispersion.
mixture of metallic Pt◦ as a main Pt species and a small amount of
oxidized Pt species, which is converted to the metallic Pt◦ by the
reduction treatment at 300 ◦C. As shown in Fig. 2B, the Fourier trans-
forms of the EXAFS for the as-prepared and reduced Pd@Pt/PVP/C show
significantly lower peak height for the Pt-Pt (and/or Pt-Pd) coordination
than that for Pt foil, indicating that metal particle sizes of these samples
are small. The EXAFS curve-fitting analysis (Table 2) of the as-prepared
Fig. 2A shows the Pt L3 edge XANES spectra of Pd@Pt/PVP/C before
and after pre-reduction and Pt foil (reference compound for Pt metal).
The peak height of the as-prepared Pd@Pt/PVP/C is slightly higher than
that of Pt foil, and the reduction treatment decreased the peak height.
This suggests that Pt species in the as-prepared Pd@Pt/PVP/C is a
–
–
and reduced Pd@Pt/PVP/C shows the presence of Pt Pt and Pt Pd
bonds with bond distance (2.71–2.73 Å) that is close to that of Pt foil
(2.7X Å). Present fitting result at k/R space in Fig. S5. Since the coor-
dination number of Pt–Pt shell (4.7) is adequately higher than that of
3

S.S. Poly et al.
Applied Catalysis A, General 619 (2021) 118158
Fig. 3. Catalyst reuse for the synthesis promoted by Pd@Pt/PVP/C under the standard reaction conditions.
Fig. 4. DF-STEM-EDS mapping images (A-D), and DF-STEM image (E) with Pd shown in green and Pt in red, of Recycle-Pd@Pt/PVP/C NPs synthesized by the
flow process.
Pt–Pd shell (2.3), Pd is supposed to be covered by Pt metal, giving the
core-shell structure even after the H2 treatment [80]. The result in-
dicates that majority of Pt atoms are surrounded by Pt and a small part of
Pt atoms are next to Pd atoms, and this local structure does not change
after the reduction treatment. This EXAFS result is consistent with the
Pd@Pt core-shell model observed by the DF-STEM-EDS mapping im-
ages. The small contribution of the Pt-Cl coordination suggests the
presence of platinum chloride as a minor Pt species. The presence of the
platinum chloride (oxidized Pt) species as a minor Pt species is consis-
tent with the XANES result. Summarizing the XANES/EXAFS and
DF-STEM-EDS results, it is shown that carbon-supported Pd@Pt
core-shell metal nanoparticles are successfully prepared by the present
flow reactor approach [79].
2.2. Catalyst screening and reaction conditions
Synthesis of 2,4,6-triphenyl pyrimidine by the reaction of 1-phenyl-
ethanol, benzyl alcohol and benzamidine [26] was investigated as a
model reaction for the dehydrogenative cross-coupling of alcohols to
form pyrimidines.
Table 3 shows the catalyst screening results for the synthesis of 2,4,6-
triphenyl pyrimidines with the reaction of 1.25 mmol of 1-phenyletha-
nol, 1.5 mmol of benzyl alcohol, and 1.0 mmol of benzamidine reflux-
ing in toluene for 24 h. We compared various monometallic and
bimetallic core-shell catalysts in the terms of the yield of the pyrimidine
product based on benzamidine. The catalytic tests for the monometallic
catalysts (Pt/C and Pd/C) were carried out using 1 mol% of the catalyst.
Pt/C and Pd/C with metal content of 5 wt% show the pyrimidine yield of
4

S.S. Poly et al.
Applied Catalysis A, General 619 (2021) 118158
Scheme 2. Synthesis of pyrimidines from various secondary alcohols, primary alcohols and amidines. Isolated yields are shown.
Scheme 3. Gram scale synthesis of pyrimidines from 1-phenyl ethanol, benzyl alcohol and benzamidine.
95% and 42%, respectively (entries 1–2), while Pt/C and Pd/C with
metal content of 2.5 wt% showed lower yields of 68% and 17% (entries
3–4). The catalytic tests for the bimetallic catalysts (Pd@Pt/C, Pd@Pt/
PVP/C) were carried out using 0.5 mol% of the catalyst. The reaction by
0.5 mol% core-shell Pd@Pt catalysts prepared by the flow process
showed 93% yields (entry 6) comparable to the yield by 1 mol% Pt/C,
suggesting that catalytic efficiency is improved by the presence of Pd
core. The Pd@Pt catalyst prepared by a batch process (entry 5) showed
lower yield than that prepared by the flow process (93%) (entry 6). The
lower activity of former catalyst might be due to its larger metal particle
size (about 5.0 nm). PVP is widely used as a capping agent that plays an
important role to prevent the aggregation of nanoparticles [81,82]. The
core-shell Pd@Pt catalyst without using PVP (entry 7) showed lower
yield (84%) than that with PVP (93%).
recovered Pd@Pt/PVP/C catalyst showed high yield (93–78%) at least 5
cycles. The reaction rate slightly decreased during the recycle tests,
which is consistent with the previously results for the recycle tests with
Pt/C catalyst [26].
To investigate the possible reasons for the gradual catalyst deacti-
vation, the recycled catalyst was characterized using DF-STEM (Fig. 4).
The result revealed that the particles size for the recycled Pd@Pt/PVP/C
catalyst (3.72 ± 0.73 nm) is close to that for the fresh catalyst
(3.55 ± 0.62 nm) (Figs. S2 and S6). Comparison of the mapping images
of the fresh (Fig. 1C) and recycled (Fig. 4C) catalyst shows that the core-
shell structure is slightly changed by the reaction treatment, which can
result in the slight decrease in the activity of the recycled catalyst. The
reason of specific change is still unknown but one possibility is due to the
slightly mixing of Pd and Pt during different treatments of the catalyst. It
is noticed that the particle size of the NPs slightly increased during the
recycle study. It might be the aggregation of NPs as it seems the sec-
ondary particles are slightly higher than primary particles.
It is well known that a bimetallic core-shell structure with an inner
core of a metal and an external shell of another metal may provide
unique physical and chemical properties [83,84]. In the Pd@Pt
core-shell catalyst, Pt is slightly electron rich [85]. This might accelerate
–
Pt-catalyzed C H bond dissociation step as an important step of the
2.4. Performance of Pd@Pt/PVP/C-catalyzed dehydrogenation reaction
present pyrimidine synthesis [26].
Under the optimized reaction conditions, we studied the synthesis
pyrimidines from various secondary alcohols, primary alcohols and
amidines. As shown in Scheme 2, various primary alcohols including
aromatic and aliphatic alcohols, 1-phenyl ethanol with different sub-
stituents at para-positions, and amidines were coupled to yield the
corresponding pyrimidines (4a–4h) in good to high yields (65–93%). It
should be noted that products 4c and 4d are important intermediates in
2.3. Recyclability test of Pd@Pt/PVP/C
Then, we tested the recyclability of this catalytic system (Fig. 3).
After the standard reaction for 24 h, the catalysts were separated from
the reaction mixture by centrifugation and dried at 40 ◦C for 3 h under
vacuum, followed by the reduction in H2 at 300 ◦C for 0.5 h. The
5

S.S. Poly et al.
Applied Catalysis A, General 619 (2021) 118158
Scheme 4. Control experiments for the multicomponent synthesis of pyrimidines.
Scheme 5. Plausible reaction mechanism.
the total synthesis of rosuvastatin, which is used as a pharmaceutical
drug for treatment of patients with high levels of cholesterol [26,86].
Notably, tetra-substituted pyrimidine derivatives were also synthesized
in good yields by this protocol (4g and 4h).
of base, both alcohols reacted very slowly and produced 5% of chalcone
(Scheme 4C), suggesting that base plays an important role such as
deprotonation of alcohols and aldol condensation. The reaction of
benzamidine with chalcone in the presence of catalyst and base gave the
pyrimidine product in 95% yield (Scheme 4D). The reaction of chalcone,
prepared by the reaction of benzyl alcohol and 1-phenylethanol (4h),
with benzamidine gave the pyrimidine in 90% yield (Scheme 4E).
The dehydrogenative synthesis of pyrimidine with alcohols and
amidines with small amount of Pd@Pt/PVP/C (0.02 mol %) resulted in
72% yield of the product, corresponding to TON of 3600 (Scheme 3).
This demonstrates the high catalytic efficiency of the present system.
2.6. Reaction pathway
2.5. Control experiment
On the basis of control experiments and literature reports [26,50] a
possible pathway for the formation of substituted pyrimidines is pro-
posed in Scheme 5. The alcohols undergo dehydrogenation by a Pd@Pt
core-shell catalyst to give ketone 1a’ and aldehyde 2a’, and molecular
hydrogen. A subsequent base-mediated aldol condensation afford an
Finally, we performed control experiments to explore the plausible
reaction mechanism for the formation of 2,4,6-trisubstituted pyrimi-
–
dines via selective C C and C–N bond formation reaction catalyzed by
Pd@Pt/PVP/C catalyst. First, the cross coupling of benzyl alcohol and 1-
phenylethanol was carried out under the standard reaction conditions
(Scheme 4A). The reaction for 4 h gave chalcone in >95% yield, whereas
in the absence of catalyst, both alcohols remained unreacted (Scheme
4B). This result suggested that the dehydrogenation of alcohols was
catalyzed by Pd@Pt/PVP/C where Pt sites acts as an active site of
dehydrogenation and deprotonation steps of the reaction. In the absence
α
,β-unsaturated ketone intermediate 5a, which in turn reacts with the
amidine to give intermediate 6a via intermolecular condensation.
Finally, dehydrogenation of 6a produces the desired product 4a. The
generation of 1a’, 2a’ and 5a was confirmed using GC–MS.
6

S.S. Poly et al.
Applied Catalysis A, General 619 (2021) 118158
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Carbon supported Pd@Pt core-shell bimetallic nanoparticle catalysts
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